As is well known, a steam turbine including a casing, a shaft body (rotor) rotatably installed in the casing, a plurality of stationary blades fixed to an inner circumference of the casing and disposed in a circumferential direction, and a plurality of moving blades radially installed at the shaft body in a downstream side of the plurality of stationary blades is used. In the case of an impulse turbine among steam turbines, pressure energy of steam is converted into velocity energy by the stationary blades, and the velocity energy is converted into rotational energy (mechanical energy) by the moving blades. In addition, in the case of a reaction turbine among the steam turbines, pressure energy is converted into velocity energy in the moving blades, and velocity energy is converted into rotational energy (mechanical energy) by a reaction force caused by ejection of steam.
In the steam turbine, in general, a space in a radial direction is formed between a tip part of the moving blade and the casing configured to form a flow path for steam by surrounding the moving blade, and a space in a radial direction is also formed between a tip part of the stationary blade and the shaft body. Here, leakage steam passing through the space of the tip part of the moving blade at a downstream side does not apply a rotational force to the moving blade. In addition, leakage steam passing through the space of the tip part of the stationary blade at a downstream side barely applies a rotational force to the moving blade at the downstream side, because pressure energy is not converted into velocity energy by the stationary blade. Accordingly, in order to improve performance of the steam turbine, it is important to reduce the amount of leakage steam passing through the space.
In the following Patent Document 1, a seal structure in which a stepped part having a height gradually increased from an upstream side toward a downstream side in an axial direction is formed at a tip part of a moving blade, and in which a seal fin is formed at a casing to form a space between the seal pin and the stepped part is disclosed.
According to the seal structure, leakage flow passing through the space between the stepped part and the seal fin collides with a step surface that forms an end edge of the stepped part, increasing the flow resistance and thus reducing the leakage flow rate.